Bifunctional metamaterials with simultaneous and independent manipulation of thermal and electric fields

Biphysical undetectable concentrators manipulating both heat flux and direct current via topology optimization

Recent remarkable developments in metamaterials and metadevices manipulating diffusive processes, such as thermal and electrical conduction, have enabled the control of multiple phenomena and the development of multifunctional devices. However, only either multiphysics operations or multiple functionalities are usually implemented on single metadevices. In this paper, we describe a method for the optimal design of metadevices that achieves both cloaking and focusing in the control of both heat flux and direct current by a single device, i.e., biphysical-bifunctional metadevices having four capabilities. Our design scheme performs well in terms of providing cloaking and focusing bifunctionality. Additionally, it assumes bulk natural materials without the use of metamaterials, which improves the manufacturability of the designed metadevices. Moreover, multidirectional metad evices are optimally designed for thermal-electrical conductions transmitted from multiple directions or from heat and voltage sources at various locations.

Electromagnetic-acoustic biphysical cloak designed through topology optimization

Various strategies have been proposed to achieve invisibility cloaking, but usually only one phenomenon is controlled by each device. Cloaking an object from two different waves, such as electromagnetic and acoustic waves, is a challenging problem, if not impossible, to be achieved using transformation theory and metamaterials, which are the major approaches in physics. Here, by developing topology optimization for controlling both electromagnetic and acoustic waves, we present a multidisciplinary attempt for designing biphysical cloaks with triple-wave cloaking capabilities, specifically for E_z- and H_z-polarized waves and acoustic wave. The topology-optimized biphysical cloak cancels the scattering of the three waves and reproduces the original propagating waves as if nothing is present, thus instilling the desired cloaking capability. In addition, we describe cloaking structures for multiple incident directions of the three waves and structures that work for both electromagnetic waves and sound waves of different wavelengths.

A Continuously Tunable Solid-Like Convective Thermal Metadevice on the Reciprocal Line

The emerging thermal metamaterials and metadevices demonstrate significant potential to transform thermal conduction. However, the thermal conductivities of existing devices are all restricted at fixed values if the configuration or constituent materials are static. Thermal convection provides an additional tool to boost and flexibly modify the heat transfer in moving matter, but it is essentially distinct from thermal conduction since the Onsager reciprocity is generally broken in the former but preserved in the latter. Therefore, it is difficult to use convective components for sophisticated control of conductive heat. Here, it is shown that a convective system can be made undistinguishable from a conductive one in principle, by discovering and operating on the reciprocal line of mechanically rotating systems. The realized thermal metadevice can thus mimic a solid-like material whose thermal conductivity dynamically covers a wide range. It offers great possibilities of real-time smooth control over heat transfer for broad applications.

Effective medium theory for thermal scattering off rotating structures

Controlling heat transfer with artificial functional materials has been a promising route towards the efficient and smart utilization of thermal energy in modern society. At the macroscopic scale, thermal metamaterials have demonstrated versatile functionalities in manipulating thermal conduction. One major method is the effective medium theory, which provides a reliable approximation for the material parameters of the composite. Although most of thermal metamaterials use static components, recent devices with integrated moving parts are attracting great interest thanks to their high efficiency and flexibility. However, the effective medium theory for thermal scattering off such devices has not been well established, due to the fundamental difference between thermal convection and conduction. Here, we provide a thorough study on heat transfer through mechanically rotating structures. It is shown that the effective thermal conductivity of a rotating structure can be rigorously described in a complex plane. The analytical expressions of the effective thermal conductivity for structures with rotating multiple layers are formulated, which explicitly capture their influences on the surrounding temperature field. We validate the theory and numerically demonstrate the rotated and unrotated temperature distributions generated around a single structure. Our theory is expected to become a design recipe for novel thermal metamaterials and meta-devices.

Surface transformation multi-physics for controlling electromagnetic and acoustic waves simultaneously

A multi-physics null medium that performs as a perfect endoscope for both electromagnetic and acoustic waves is designed by transformation optics, which opens a new way to control electromagnetic and acoustic waves simultaneously. Surface transformation multi-physics, which is a novel graphical method to design multi-physics devices, is proposed based on the directional projecting feature of a multi-physics null medium. Many multi-physics devices, including beam shifters, scattering reduction, imaging devices and beam steering devices, for both electromagnetic and acoustic waves can be simply designed in a surface-corresponding manner. All devices designed by surface transformation multi-physics only need one homogeneous anisotropic medium (null medium) to realize, which can be approximately implemented by a brass plate array without any artificial sub-wavelength structures. Numerical simulations are given to verify the performances of the designed multi-physics devices made of brass plate array.

Manipulating cell: flexibly manipulating thermal and DC fields in arbitrary domain

To extend the metamaterial applications on simultaneously regulating multiple fields with transformation optics, we propose a class of manipulative cell here to manipulate thermal and DC fields simultaneously in non-conformal angular schemes. Significant behaviors of thermal cloaking, electrical concentration, and related switched functions are numerically demonstrated with appropriate media. The findings not only present an efficient method for simultaneously manipulating various energy, but also break the limitation of structural profiles in the designs of bi-functional meta-device. Moreover, it may also provide references for efficient energy manipulation and management in conventional energy techniques.

Multiphysical Digital Coding Metamaterials for Independent Control of Broadband Electromagnetic and Acoustic Waves with a Large Variety of Functions

Fabricating materials with customized characteristics for both electromagnetic (EM) and acoustic waves remain a significant challenge using the current technology, since the demand of multiphysical manipulation requires a variety of material parameters that are hard to satisfy in nature. However, the emergence of artificially structured materials provides a new degree of freedom to tailor the wave-matter interactions in dual physical domains at the subwavelength scale. Here, a bifunctional digital coding metamaterial (MM) is proposed to engineer the propagation behaviors of EM and acoustic waves simultaneously and independently. Four kinds of rigid pillars with various material properties are employed to serve as 1-bit reflection-type digital meta-atoms with antiphase responses in both frequency spectra, thus offering the opportunities for independent field control as desired. The MM demonstrates excellent performance of scattering manipulations from 5700 to 8000 Hz in the acoustic region and 5.80-6.15 GHz in the microwave region. The bifunctional MM is verified through full-wave simulations and experimental measurements with good agreement, which stands out as a powerful tool for related applications in the future.

Transparent coupled membrane metamaterials with simultaneous microwave absorption and sound reduction

Metamaterials offer a novel strategy to control wave propagation in different physical fields ranging from acoustic, electromagnetic, and optical waves to static electric and thermal fields. However, fundamental and practical challenges still need to be overcome for multi-physical manipulation, especially for independent control of acoustic and electromagnetic waves simultaneously. In this paper, we propose and experimentally demonstrate a transparent bifunctional metamaterial in which acoustic and electromagnetic waves could be engineered jointly and individually. Specifically, a transparent composite coupled membrane metamaterial is introduced with indium tin oxide (ITO) patterns coated on the top and bottom membranes, giving rise to simultaneous electromagnetic wave dissipation and sound reduction. Our results could help broaden the current research scope for multiple disciplines and pave the way for the development of multi-functional devices in new applications.